Both forecast and forecasted are widely used as the past tense and past participle of the verb forecast, but the uninflected form is more common. In 21st-century English it prevails by a large margin, but not by such a large margin that anyone should consider forecasted wrong. The ratio of the past-tense and participial forecast to forecasted in 21st-century newswriting is about 20 to one, and it’s five to one in books (where forecasted is disproportionately common in financial writing) and two to one in scientific and scholarly writing.
The second syllable of forecast derives from the earlier verb cast, which has usually gone uninflected throughout its eight centuries in English. But forecasted has always been more common than casted (relative to forecast and cast, respectively) and is accepted to a degree that casted has never attained outside specialized uses. Treating forecast the same as cast might seem logical, but English usage isn’t guided by logic.
Both forms are easily found in diverse types of writing. Here are a few examples:
Clearly, we would be happiest if the two forcasting methods have forecast errors with large negative correlation. [Introduction to Time Series Analysis and Forecasting]
Many forecasted that Mitt Romney would defeat President Obama in the 2012 presidential election. [Guardian]
[T]he forecasted demand for bioartificial organs—cellular, autograft, allograft, and xenograft transplants—is estimated to be in the billions of dollars per annum. [Nature]
The company’s analysts had forecast it would take as long as nine months to find that many qualified engineers in the United States. [New York Times]
To demonstrate his orthodoxy and his attachment to the Church and its triumph he composed works in which he forecasted its universal greatness. [History of Italian Philosophy, Eugenio Garin]
But at the same time, other studies have forecast that warmer temperatures will reduce the wind shear necessary to turn a routine thunderstorm into a powerful system that can give birth to tornadoes. [Time]